Glow plugs/cold start
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Glow plugs/cold start
My 1987 F-250 will not start in the morning. What really puzzles me is how this system/glow plugs work. I measured resistance between the glow plug harness and ground and read 0 ohms. I took one of the plugs out and measured between the where it connects to the harness and the body where it screws into the engine and read 0 ohms. I went to two auto parts stores and measured the resistance of three glow plugs between the interface to the harness and the body where it mounts to the engine and read 0 ohms. How can this system heat with a direct short to the engine?
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From: Virginia
It's not a complete short, but close to it. That's how it works to heat the glowplug. Someone on here said the whole system draws around 200 amps when it's activated. I would guess also, the resistance might increase a little bit when they get hot.
Post Fiend
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From: Nutter Fort, WV
You need a better (100 dollar range digital meter) to be able to get good readings on glow plug ohms.
A good plug should be between .5 and 1.5 ohms, which is almost a dead short.
Most cheap meters will just flash 0 ohms when the resistance is that low.
A good plug should be between .5 and 1.5 ohms, which is almost a dead short.
Most cheap meters will just flash 0 ohms when the resistance is that low.
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If I understand you guys, the resistance from the power interface (connection to the harness) of the plug to the block should be near 0??? I'm not talking about across the plug. It just seems to me that the power interface should be isolated from the engins block or the grounded block will sink the current (the current will travel into the block instead of to the heating element in the tip of the plug). What is wrong with my thinking?
Thank you all for your input. With all this information I'm gathering, I'll soon zero in on the solution.
Thank you all for your input. With all this information I'm gathering, I'll soon zero in on the solution.
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Okay, I just got it. When I measure from the power-in interface to the case, I'm actually measuring across the heating element. Sorry it took so long to sink in. Thanks to all.
But now I have to figure out what else is wrong.
It's an '87 - 6.9L
But now I have to figure out what else is wrong.
It's an '87 - 6.9L
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Post Fiend
Joined: Nov 2003
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From: Nutter Fort, WV
The glow plug case with the threads on it is attached to one end of the heating element inside the glow plug.
The connector is attached to the other end of the heating element.
The tip of the glow plug where the wire is attached is isolated from the case, just like the old style lightbulbs with brass bases were.
All connections between the glow plug, harness, relay and even the battery should have no resistance in the connections.
However, the wire size does come into play to regulate the amount of amps that get to the glow plugs through voltage drop, so don't change the wire size trying to get less resistance between the battery and the glow plugs or you will have toasted glow plugs in a very short amount of time.
At 12 volts, .5 ohms resistance means the glow plug draws 24 amps or 288 watts of power to heat the element.
The connector is attached to the other end of the heating element.
The tip of the glow plug where the wire is attached is isolated from the case, just like the old style lightbulbs with brass bases were.
All connections between the glow plug, harness, relay and even the battery should have no resistance in the connections.
However, the wire size does come into play to regulate the amount of amps that get to the glow plugs through voltage drop, so don't change the wire size trying to get less resistance between the battery and the glow plugs or you will have toasted glow plugs in a very short amount of time.
At 12 volts, .5 ohms resistance means the glow plug draws 24 amps or 288 watts of power to heat the element.
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From: Joliet, Illinois
Today I tested 8 unused/new glowplugs I had left over from my dealership days and they tested from a low of 1.3 ohms all the way up to a maximum of 2.0 ohms with a few in the 1.7 range.
I was going to test the ones on the truck as it smokes when started cold, but the plastic end just crumbled as I touched it!
Adding 8 glow plug connector ends to the shopping list!
I was going to test the ones on the truck as it smokes when started cold, but the plastic end just crumbled as I touched it!
Adding 8 glow plug connector ends to the shopping list!
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I'll tell you, you fellows are good. I learned a good bit by reading this. Great job!!!! Many thanks!
Yeah I know what you mean about the cost to replace the harness. I called Ford to purchase a new one - $139.....................for a harness that is essentially 6 feet of insulated wire with about 3 more feet attached????????? I was thinking maybe $20 with a cost to manufacture of $1.5 or less. Maybe the wires are made of gold.
Can you buy glow plug connector ends? I can get teflon insulated wire and make a new cable.
Yeah I know what you mean about the cost to replace the harness. I called Ford to purchase a new one - $139.....................for a harness that is essentially 6 feet of insulated wire with about 3 more feet attached????????? I was thinking maybe $20 with a cost to manufacture of $1.5 or less. Maybe the wires are made of gold.
Can you buy glow plug connector ends? I can get teflon insulated wire and make a new cable.
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From: Joliet, Illinois
Originally Posted by Lionbacker
Can you buy glow plug connector ends? I can get Teflon insulated wire and make a new cable.
If not, I'll just have to fab something up.
Yesterday I used 1/8 brass rod, threaded on one end and using brass nuts on the inside to replace the front tank sender connection into the fuel tank, but I'm not sure, but I think I might have reversed the polarity as the wiring end connector looked like it would rather have been facing the other way and since the end was found dangling attached to the end connector, I had no idea which way it went.
It took half the day to R+R the tank and repair the sender.
But I do have more time than money right now.
Post Fiend
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From: Nutter Fort, WV
I can't remember the wire gauge off the top of my head, but it is something like 10 AWG, one lead for each cylinder bank.
Given the amount of amperage that the glow plugs use, that wire is way to small for constant use at that amperage.
But since the glow plugs are only used for 10 seconds or so, the wire just winds up limiting the amount of amperage that it can carry.
Several people have used heavier wire to construct the harness, they all had problems with burning up the glow plugs even at 10 seconds of glow time.
Given the amount of amperage that the glow plugs use, that wire is way to small for constant use at that amperage.
But since the glow plugs are only used for 10 seconds or so, the wire just winds up limiting the amount of amperage that it can carry.
Several people have used heavier wire to construct the harness, they all had problems with burning up the glow plugs even at 10 seconds of glow time.